Food doesn't last.
食物不能长期保存。
In days, sometimes hours, bread goes moldy, apple slices turn brown, and bacteria multiply in mayonnaise.
在几天甚至几小时后面包就会发霉，切片的苹果变黄，细菌在蛋黄酱里滋生。
But you can find all of these foods out on the shelf at the grocery store, hopefully unspoiled, thanks to preservatives.
但是你在超市货架上都可以看到这些食品，希望还没坏，这要感谢防腐剂。
But what exactly are preservatives?
但是到底什么是防腐剂？
How do they help keep food edible and are they safe?
它们怎么让食物维持可食状态又安全？
There are two major factors that cause food to go bad: microbes and oxidation.
有二种主要原因造成食物腐败，微生物及氧化。
Microbes like bacteria and fungi invade food and feed off its nutrients.
微生物如细菌及霉菌会侵袭食物，靠着食物的养分维生。
Some of these can cause diseases, like listeria and botulism.
有些细菌会造成疾病，例如李斯特菌及肉毒杆菌。
Others just turn edibles into a smelly, slimy, moldy mess.
其它则让食品变成又臭又黏的霉菌团。
Meanwhile, oxidation is a chemical change in the food's molecules caused by enzymes or free radicals
氧化作用则是食物分子产生化学变化，酵素或自由基造成脂肪酸败，
which turn fats rancid and brown produce, like apples and potatoes.
或使农产品如苹果、马铃薯褐变。
Preservatives can prevent both types of deterioration.
防腐剂可以避免这两种腐败现象。
Before the invention of artificial refrigeration, fungi and bacteria could run rampant in food.
在冰箱发明之前，真菌及细菌可在食物内滋生。
So we found ways to create an inhospitable environment for microbes.
所以我们找到了几个方法，创造对微生物不利的环境。
For example, making the food more acidic unravels enzymes that microbes need to survive.
例如，让食物更酸，会破坏微生物生存所需的酵素。
And some types of bacteria can actually help.
某些细菌其实在这一点上还有帮助。
For thousands of years, people preserved food using bacteria that produce lactic acid.
几千年来大家利用会产生乳酸的微生物来保存食物。
The acid turns perishable vegetables and milk into longer lasting foods,
乳酸将易腐败的蔬菜及牛奶转变成可以长久保存的食物，
like sauerkraut in Europe, kimchi in Korea, and yogurt in the Middle East.
像是欧洲的德式酸白菜，韩国的泡菜及中东地区的酸奶。
These cultured foods also populate your digestive track with beneficial microbes.
这些发酵食品也能使肠道内的益生菌繁殖。

Many synthetic preservatives are also acids.
许多人工防腐剂也是酸类。
Benzoic acid in salad dressing, sorbic acid in cheese, and propionic acid in baked goods. Are they safe?
沙拉酱里的安息香酸，芝士里的己二烯酸以及烘培食品内的丙酸都是。它们安全吗？
Some studies suggest that benzoates, related to benzoic acid, contribute to hyperactive behavior.
某些研究显示，安息香酸盐，即安息香酸的相关物，可能会造成过动行为。
But the results aren't conclusive.
但是目前还没有定论。
Otherwise, these acids seem to be perfectly safe.
如果不是的话，这些酸类似乎非常安全。
Another antimicrobial strategy is to add a lot of sugar, like in jam, or salt, like in salted meats.
其它的抗菌策略还有添加很多糖，像是果酱，或很多盐，像腌肉。
Sugar and salt hold on to water that microbes need to grow
糖和盐会抓住微生物生长所需的水分，
and actually suck moisture out of any cells that may be hanging around, thus destroying them.
也就是将在糖盐周围的细胞水分吸干，因而摧毁它们。
Of course, too much sugar and salt can increase your risk of heart disease, diabetes, and high blood pressure,
当然，太多糖及盐会增加心脏病、糖尿病及高血压的风险，
so these preservatives are best in moderation.
所以要适量使用这种防腐剂。
Antimicrobial nitrates and nitrites, often found in cured meats,
抗菌的硝酸及亚硝酸盐常常可在腌肉内找到，
ward off the bacteria that cause botulism, but they may cause other health problems.
以防止肉毒症细菌滋生，但是这也会造成其它健康问题。
Some studies linking cured meats to cancer have suggested that these preservatives may be the culprit.
有些研究显示腌肉之所以致癌，这种防腐剂可能是罪魁祸首。
Meanwhile, antioxidant preservatives prevent the chemical changes that can give food an off-flavor or color.
抗氧化剂也可以防止造成食物变味或变色的化学变化。
Smoke has been used to preserve food for millennia
烟熏也是过去数千年来一直使用的食物保存方法，
because some of the aromatic compounds in wood smoke are antioxidants.
因为木头熏烟内的某些芳香族化合物是抗氧化剂。
Combining smoking with salting was an effective way of preserving meat before refrigeration.
烟熏加上盐腌是冰箱发明以前保存肉类的有效方法。
For antioxidant activity without a smoky flavor, there are compounds like BHT and tocopherol, better known as vitamin E.
如果你没有烟熏口味的抗氧化剂，可用化合物如二丁基羟基甲苯及生育酚，也就是大家熟知的维生素E。
Like the compounds in smoke, these sop up free radicals
它们就像熏烟内的化合物，可以吸干自由基，
and stave off rancid flavors that can develop in foods like oils, cheese, and cereal.
避免出现在油类、芝士及早餐谷片内的酸败的味道，
Other antioxidants like citric acid and ascorbic acid
其它抗氧化剂，如柠檬酸及维他命C，
help cut produce keep its color by thwarting the enzyme that causes browning.
有助于切片农产品保持颜色，因为它会阻碍褐变酵素作用。
Some compounds like sulfites can multitask. They're both antimicrobials and antioxidants.
像亚硫酸盐这样的某些化合物有多重功能。它们可同时作为抗菌剂及抗氧化剂。
Sulfites may cause allergy symptoms in some people, but most antioxidant preservatives are generally recognized as safe.
亚硫酸盐可能会让某些人产生过敏症状，但是大多数抗氧化型防腐剂通常被认为是安全的。
So should you be worried about preservatives?
所以我们应该担心防腐剂吗？
Well, they're usually near the end of the ingredients list
这么说吧，它们通常位于成分表的最后几项，
because they're used in very small amounts determined by the FDA to be safe.
因为用量非常少，美国食药局认定其为安全的。
Nevertheless, some consumers and companies are trying to find alternatives.
尽管如此，一些消费者及公司还是在寻找替代品。
Packaging tricks, like reducing the oxygen around the food can help,
包装技巧，如减低食物周围的氧气，也有所些作用
but without some kind of chemical assistance, there are very few foods that can stay shelf stable for long.
但是没有一定程度的化学帮助，很少有食物能在常温下贮藏很久。